diff --git a/hw/core/qdev-properties-system.c b/hw/core/qdev-properties-system.c
index 9aa80495ee..3f84309b7e 100644
--- a/hw/core/qdev-properties-system.c
+++ b/hw/core/qdev-properties-system.c
@@ -25,6 +25,28 @@
 #include "sysemu/iothread.h"
 #include "sysemu/tpm_backend.h"
 
+static bool check_prop_still_unset(DeviceState *dev, const char *name,
+                                   const void *old_val, const char *new_val,
+                                   Error **errp)
+{
+    const GlobalProperty *prop = qdev_find_global_prop(dev, name);
+
+    if (!old_val) {
+        return true;
+    }
+
+    if (prop) {
+        error_setg(errp, "-global %s.%s=... conflicts with %s=%s",
+                   prop->driver, prop->property, name, new_val);
+    } else {
+        /* Error message is vague, but a better one would be hard */
+        error_setg(errp, "%s=%s conflicts, and override is not implemented",
+                   name, new_val);
+    }
+    return false;
+}
+
+
 /* --- drive --- */
 
 static void get_drive(Object *obj, Visitor *v, const char *name, void *opaque,
@@ -299,14 +321,16 @@ static void set_netdev(Object *obj, Visitor *v, const char *name,
     }
 
     for (i = 0; i < queues; i++) {
-
         if (peers[i]->peer) {
             err = -EEXIST;
             goto out;
         }
 
-        if (ncs[i]) {
-            err = -EINVAL;
+        /*
+         * TODO Should this really be an error?  If no, the old value
+         * needs to be released before we store the new one.
+         */
+        if (!check_prop_still_unset(dev, name, ncs[i], str, errp)) {
             goto out;
         }
 
diff --git a/hw/core/qdev-properties.c b/hw/core/qdev-properties.c
index ead35d7ffd..71f8aca7c6 100644
--- a/hw/core/qdev-properties.c
+++ b/hw/core/qdev-properties.c
@@ -1246,6 +1246,23 @@ void qdev_prop_register_global(GlobalProperty *prop)
     g_ptr_array_add(global_props(), prop);
 }
 
+const GlobalProperty *qdev_find_global_prop(DeviceState *dev,
+                                            const char *name)
+{
+    GPtrArray *props = global_props();
+    const GlobalProperty *p;
+    int i;
+
+    for (i = 0; i < props->len; i++) {
+        p = g_ptr_array_index(props, i);
+        if (object_dynamic_cast(OBJECT(dev), p->driver)
+            && !strcmp(p->property, name)) {
+            return p;
+        }
+    }
+    return NULL;
+}
+
 int qdev_prop_check_globals(void)
 {
     int i, ret = 0;
diff --git a/include/hw/qdev-properties.h b/include/hw/qdev-properties.h
index 5252bb6b1a..fc0b2f6b45 100644
--- a/include/hw/qdev-properties.h
+++ b/include/hw/qdev-properties.h
@@ -251,6 +251,8 @@ void qdev_prop_set_macaddr(DeviceState *dev, const char *name,
 void qdev_prop_set_enum(DeviceState *dev, const char *name, int value);
 
 void qdev_prop_register_global(GlobalProperty *prop);
+const GlobalProperty *qdev_find_global_prop(DeviceState *dev,
+                                            const char *name);
 int qdev_prop_check_globals(void);
 void qdev_prop_set_globals(DeviceState *dev);
 void error_set_from_qdev_prop_error(Error **errp, int ret, DeviceState *dev,